Transparent conductive oxide (TCO) films of materials such as ITO, tin oxide, cadmium oxide, and the like are known in the art. Although there are many materials that are either (a) electrically conductive or (b) optically substantially transparent in the visible, TCOs exhibit a useful compromise of both these desirable properties. The metric that encodes this combination of properties is the ratio of σdc/σopt. According to electromagnetic theory, high conductivity and optical transparency are mutually exclusive properties. This is due to the fact that when there is a large density of charge carriers available to give rise to high electrical conduction; the incoming electromagnetic radiation is dissipated due to skin effect of the free charge carriers or simply reflected if the frequency of excitation is too high for the carriers to follow.
In semi-conducting oxide films such as tin oxide and indium oxide, good transparency and electrical conductivity can be obtained by adding dopants that enter substitutionally into the lattice or by adjusting the preparation condition, thereby obtaining a controlled non-stoichiometry of the material. Since the doping is done on an insulator whose band gap is greater than 3 eV, the material transmits most part of the visible radiation. Doping leads to two categories of transparent conducting oxides (TCOs), namely p-type and n-type. Binary compounds such as tin oxide, indium oxide and zinc oxide are n-type dopable transparent conducting oxides. CuAlO2, CuGaO2 and SrCu2O are p-type TCO compounds. The fact that these high performance TCOs rely on (i) binary, ternary or quarternary alloys and (ii) on controlled doping make their high temperature processing in ambient conditions very difficult. It is known in the art that a permanent thin dielectric barrier over the TCO can improve thermal processability to about 350-400 C, but this solution makes electrical contact to the film non-viable for most applications.
Commonly used as TCO electrodes in many opto-electronics applications, In2O3 and In2O3:SnO2 (a form of ITO) are known wide band-gap oxide semiconductors (3.79 eV in bulk form). At low concentration of extrinsic dopant (e.g., SnO or Sn dopes indium oxide; or F does tin oxide), charge carriers are mainly attributed to oxygen vacancy defects, such as: Oo→½O2+V2++2e. Tin doping of In2O3 films to form ITO, for example, has been used to dramatically improve electrical conductivity where the substitution of two tin atoms and one oxygen interstitial form a charge neutral carrier site in addition to the intrinsic oxygen vacancy sites. A TCO in this respect is a degenerated semiconductor due to the dopant(s). Since grain boundaries act as impurity sinks, they are very relevant to electrical conduction. Therefore, changes in the grain boundary area per unit volume due to heat treatment will result in charge carrier concentration changes and eventually lead to large drifts in conductivity of the films. In addition, oxygen diffusion from the ambient atmosphere into the TCO film, as well as extrinsic dopants leaving the TCO film during HT, have the effect of adversely changing σdc/σopt.
However there are a range of applications whereby for various reasons the transparent substrates have to be heat-strengthened (e.g., glass to be thermally tempered) after TCO deposition thereon. Glass is often heat treated (HT), such as being thermally tempered, for safety and/or strengthening purposes. For example, glass substrates are often heat treated at a high temperature(s) (e.g., at least about 580 degrees C., more typically from about 600-650 degrees C.) for purposes of thermal tempering.
Accordingly, those skilled in the art will appreciate that a need in the art exists for a method of providing a heat treated (HT) coated article including an electrode where following HT the electrode still possesses sufficient electrical conductivity. A need for corresponding coated articles, both heat treated and pre-HT, also exists.